Parasitic infections are common globally, affecting millions of people each year. While some parasites cause no symptoms, others can lead to severe illness and even death. Diagnosing a parasitic infection quickly is important so that proper treatment can be started. Blood tests are one way to check for the presence of certain parasites.
What are parasites?
Parasites are organisms that live on or inside a host organism and derive nourishment from them. There are many different types of parasites, including:
- Protozoa: Single-celled organisms like Giardia, Cryptosporidium, and Toxoplasma.
- Helminths: Worms like tapeworms, pinworms, roundworms, and flukes.
- Ectoparasites: Organisms that live on the skin, like scabies and lice.
Many parasites have complex life cycles involving multiple hosts. Transmission typically occurs through contaminated food or water, contact with animals or infected persons, insect bites, or mother-to-child transmission.
What are the symptoms of parasitic infections?
Symptoms of parasitic infections vary widely depending on the type of parasite. Some common symptoms may include:
- Diarrhea
- Abdominal pain and cramps
- Nausea and vomiting
- Fatigue
- Unexplained weight loss
- Skin rashes or lesions
However, many parasitic infections may not cause any symptoms initially. Over time, chronic infections can lead to malnutrition, anemia, growth delays in children, and other problems.
Why test blood for parasites?
Blood tests allow doctors to check for the presence of parasites or antibodies made by the immune system against them. Blood tests are less invasive than other diagnostic methods like biopsies or aspiration of fluid from organs. Reasons to test blood include:
- Screen high-risk individuals like travelers, immigrants, or immunocompromised persons
- Evaluate unexplained symptoms like chronic diarrhea, fatigue, or abdominal pain
- Confirm suspected parasitic infections based on patient history and symptoms
- Follow-up after treatment to monitor response
Blood tests for parasites may be general screening tests looking for antibodies to multiple parasites or specific tests for certain infections like malaria or Chagas disease.
What blood tests detect parasitic infections?
Some examples of blood tests used to diagnose parasitic infections include:
General antibody tests
- Multiple parasite screen: Checks for immunoglobulin G (IgG) antibodies to many parasites like Toxoplasma, Strongyloides, filariasis, and amebiasis.
- Eosinophilia panel: Measures eosinophil count and antibodies to parasites that can cause increased eosinophils like hookworm, Strongyloides, and schistosomiasis.
Protozoa
- Malaria: Microscopic examination to visualize Plasmodium parasites and rapid diagnostic tests (RDTs) to detect malaria antigens.
- Trypanosomiasis: Detects Trypanosoma cruzi antibodies indicating Chagas disease.
- Toxoplasmosis: Checks for IgM and IgG antibodies to Toxoplasma gondii.
Helminths
- Echinococcosis: ELISA to detect antibodies against tapeworm Echinococcus granulosus or E. multilocularis.
- Filariasis: Identifies microscopic larvae (microfilaria) or antigens of worms like Wuchereria bancrofti, Brugia malayi, or Loa loa.
- Schistosomiasis: Checks for IgM and IgG antibodies to Schistosoma worms.
Less commonly, microscopic examination of blood smears may reveal some larger parasites like microfilaria or Trypanosoma. Molecular tests like PCR are also sometimes used to detect parasitic DNA in blood.
Benefits of blood testing
Blood testing for parasites has several advantages:
- Non-invasive: Requires only a blood draw versus more invasive tests like biopsies.
- Sensitive: Can detect even low levels of parasites or antibodies.
- Specific: Distinguishes different parasitic infections based on detected antibodies.
- Rapid: Some antibody and antigen tests give results in minutes.
- Convenient: Blood samples can be easily collected at clinics or doctor’s offices.
Limitations of blood testing
Despite their benefits, blood tests also have some disadvantages:
- Timing: May take 1-4 weeks after infection before antibodies develop, giving false negative early results.
- Persistence: Antibodies can persist for months to years after parasite clearance, complicating result interpretation.
- Cross-reactivity: Antibodies to related parasites may cause false positive results.
- Cost: Multiple or specialized antibody tests can be expensive and require advanced laboratory equipment.
- Invasiveness: Still requires blood draw which some patients dislike compared to stool or urine tests.
Therefore, blood tests are often used in combination with other diagnostic methods for the most accurate parasite detection.
Common blood tests for specific parasites
Here is more detail on some of the most common blood tests used to diagnose parasitic infections:
Malaria Blood Tests
- Microscopic examination of blood smears to visually identify Plasmodium parasites.
- Rapid diagnostic tests (RDTs) using dipsticks to detect Plasmodium antigens.
- PCR to amplify and detect Plasmodium DNA.
Both microscopy and RDTs allow rapid, inexpensive, point-of-care testing for malaria. However, microscopy requires trained personnel and has lower sensitivity with low parasite counts. RDTs are easier to perform but have some accuracy limitations. PCR is more sensitive but requires advanced laboratory capacity.
Chagas Disease Testing
Chagas disease is caused by the protozoan parasite Trypanosoma cruzi. Diagnosis relies on detecting antibodies to the parasite:
- IgG antibodies appear early and persist. Positive results indicate current or prior infection.
- IgM antibodies appear 1-2 months after infection but decline after a few months. Their presence indicates acute or recent infection.
Both ELISA and indirect immunofluorescence assays are commonly used to detect anti-T. cruzi antibodies. PCR can sometimes also detect parasitic DNA during acute infection.
Toxoplasmosis Testing
Toxoplasmosis results from infection by the parasite Toxoplasma gondii. As with Chagas disease, both IgG and IgM antibodies are measured:
- IgG antibodies appear within 1-2 weeks of infection and persist for life.
- IgM antibodies appear earlier than IgG and decline within months.
In addition to confirming diagnosis, toxoplasmosis blood tests are commonly performed in pregnant women to identify risk of congenital transmission to the fetus.
Filariasis Testing
Filariasis is caused by infection with parasitic worms like Wucheria bancrofti, Brugia malayi, or Loa loa. Diagnosis relies on detecting worm antigens or antibodies in blood:
- Microfilaria can sometimes be visualized directly on blood smears.
- Antigen tests use monoclonal antibodies to detect filarial antigens.
- Antibody tests like ELISA detect host IgG against filarial worms.
Antigen and antibody tests are more sensitive than blood smear microscopy. Antigen tests in particular are used in filariasis elimination programs for screening and monitoring.
Cysticercosis Testing
Cysticercosis results from infection with the pork tapeworm Taenia solium. Diagnosis relies on imaging and antibody detection:
- Imaging like CT or MRI can visualize larval cysts in tissues.
- ELISA for detecting antibodies to T. solium antigens in serum or CSF.
Neurocysticercosis in the brain is a particular concern. Antibody testing of CSF is helpful in diagnosing neurocysticercosis when imaging results are unclear or lesions cannot be visualized.
Should blood be tested for parasites?
Blood testing for parasites can be a useful diagnostic tool but is not necessarily appropriate in all cases. Factors in deciding whether to test blood include:
- Pre-test probability based on patient risk factors and symptoms
- Testing availability, cost, and wait times
- Whether antigen or antibody detection is desired
- Likelihood of positive blood test result for suspected parasites
- Need for microscopic vs serological vs molecular testing
Discussing the limitations and interpretation of results with patients is also important when ordering blood parasitic testing.
Conclusion
While stool examination remains the most common method to diagnose intestinal parasitic infections, blood tests play an important complementary role. Examining blood allows sensitive detection of some parasitic infections that are harder to diagnose directly through stool or other specimens. The ability to detect antibodies and antigens related to parasites also makes blood testing a useful diagnostic and epidemiological tool.
However, limitations like timing of antibody development, persistence, and cross-reactivity can complicate test result interpretation. Therefore, clinicians must understand proper use cases, benefits, and limitations of various blood tests for parasites. Ordering the correct blood tests and properly interpreting results allows early diagnosis, treatment, and monitoring of multiple parasitic infections.
